Pedicle screws are used principally for the dorsal stabilization of the spinal column in the case of fractures, tumours, infections, deformities and degenerative instabilities by means of a transpedicular screw fixing. Pedicle screws are thereby placed in the pedicles of each adjacent vertebrae, whereupon an angularly-stable connection is created between the pedicle screws which are arranged axially above each other and an axially extending longitudinal member or bar. The pedicle screws and longitudinal member thereby form a vertebra stabilization system.
For this purpose, a pedicle screw generally has an axial, shaft-shaped externally threaded portion to which a so-called tulip is connected on the screw head side. This forms constructively a U-shaped longitudinally slotted/tunnelled receiving sleeve with internal thread, wherein the two longitudinal slots lying radially opposite each other each define a slot gap of predetermined gap width. The longitudinal member is transversely inserted in the longitudinal slots which run parallel to each other, and fixed by means of a locking element, for example a grub screw or threaded nut, which is screwed into the internal thread.
In principle two basic types of pedicle screw are distinguished, namely monoaxial and polyaxial pedicle screws. In the case of a monoaxial pedicle screw, the externally threaded portion or shaft and the tulip are integrally formed with each other such that they are basically fixedly connected to each other, for example welded or soldered. By contrast a polyaxial pedicle screw has, as a separate shaft member, a manufactured externally threaded portion having a mostly spherical or (semi-)spherical screw head which is encompassed relatively pivotably and at the same time grasped from behind by the sleeve-shaped tulip, in the transition region between head and shaft. In this way, after sinking of the externally threaded portion in the pedicle channel of a vertebra, the tulip can be pivoted and/or turned relative to the shaft so as to obtain a desired position and orientation which is essentially independent of the orientation of the shaft. In this way the undercut prevents the tulip from being able to be pulled off from the shaft head. Subsequently the tulip is positionally fixed to the screw head by means of the grub screw in the case of a temporarily fixed bar (one-screw principle) or though an additional screw/nut (multi-screw principle).
Pedicle screws are put in place in the pedicle channel of a vertebra or anchored through screwing, by a surgeon. In doing so the operator aligns the screws on the basis of the orientation of the pedicle channel. When the screws are set, the above mentioned longitudinal member or bar of the correct length is selected, and optionally its longitudinal curvature is adjusted to the pedicle screws and its respective position is also adjusted. The longitudinal member should in addition be placed in the tulips of the pedicle screws.
The polyaxial pedicle screws distinguish themselves henceforth in that the tulip/receiving sleeve is at first movably placed with respect to the screw shaft such that the receiving sleeve can take on a different orientation from the shaft. This makes it easier for the surgeon to laterally insert/guide the longitudinal member in the receiving sleeve. When the surge is satisfied with the seating of the longitudinal member and the receiving sleeve, he locks the pedicle screw by means of the locking element, preferably by means of the grub screw (in principle also termed set screw of any configuration).
The polyaxial locking (positional fixing of the receiving sleeve with respect to the shaft) as well as the clamping of the longitudinal member are locked upon tightening of the set screw in a single assembly step in the case of the above-mentioned one-screw principle.
In the case of polyaxial screws it is generally not possible during the implanting procedure to introduce forces from the receiving sleeve to the vertebra due to the mobility (link) between the screw shaft and the receiving sleeve, in order to manipulate them for example. However this is required in particular with repositioning manoeuvres in the case of fractures or spondylolisthesis and partly with compressions or distractions. For this purpose therefore the monoaxial pedicle screws are used, whereby the receiving sleeve is rigid with the screw shaft. However these monoaxial pedicle screws have the disadvantage that the longitudinal member can only with difficulty be laterally introduced into the tulip/receiving sleeve of several monoaxial pedicle screws.
A polyaxial pedicle screw according to the above mentioned one-screw principle is known from the prior art, for example according to EP 2 301 458 A1, consisting of a shaft member having external thread and spherical head as well as a U-shaped longitudinally slotted receiving sleeve (tulip) for a longitudinal support/bar. The receiving sleeve has an internal thread in the axial region towards the opening of the longitudinal slots, in which a grub screw can be screwed, and a radially inwardly oriented circumferential projection or land in the axial region up to the respective slot base. Furthermore a type of piston or inlay is inserted in the receiving sleeve by means of a snap ring and in this way secured against falling out.
For the assembly of the polyaxial pedicle screw known from EP 2 301 458 A1, the receiving sleeve is first passed from the distal end (the end lying opposite the shaft head) of the shaft member over this shaft member until the radial internal land of the receiving sleeve buts against the shaft head (on its under side). Subsequently the inlay is pushed into the receiving sleeve (i.e. on the upper side of the shaft head) such that the spring ring which is arranged circumferentially between receiving sleeve and inlay snaps into corresponding circumferential grooves on the inlay and on the receiving sleeve, and axially holds both components together. The shaft head is therefore arranged between the land of the receiving sleeve and the inlay (i.e. underneath the inlay).
As soon as the pedicle screw is screwed into a vertebra and fixedly anchored into it, a longitudinal member is introduced into the U-shaped (double) slot of the receiving sleeve (above the inlay), wherein the receiving sleeve can turn and pivot relative to the anchored shaft member. In this way it is possible for an operator to adjust the receiving sleeve to correspond to the orientation of the longitudinal member. As soon as the suitable relative position of the receiving sleeve is set, the grub screw is screwed into the receiving sleeve until it locates the longitudinal member against the inlay and presses this further in the axial direction of the receiving sleeve against the shaft head. In this way the entire pedicle screw/longitudinal member system (vertebra-stabilization system) can be fixed/locked in the set position by tightening the single grub screw (locking element).
US 2008/02155100 A1 discloses for example a polyaxial pedicle screw according to the above mentioned multi-screw principle. This pedicle screw also has a shaft-shaped externally threaded portion having an integral shaft head at a proximal end of the shaft. The shaft head is freely rotatably and pivotably surrounded by a receiving sleeve, in which an internal thread is also configured and which comprises two U-shaped longitudinal slots which lie opposite each other, for a longitudinal member.
A piston/inlay is axially movably inserted in the receiving sleeve, which likewise comprises a U-shaped longitudinal slot with the approximate slot width dimension of the longitudinal slots in the receiving sleeve. The flanks that occur here at the U-shaped longitudinal slot of the inlay are elongated in the axial direction such that they protrude over a bar/longitudinal member transversely inserted in it. In other words the length of the flanks of the inlay is larger than the diameter of the transversely inserted bar/longitudinal member.
In order to assemble the pedicle screw known from US 2008/02155100 A1, the receiving sleeve/tulip is passed over the shaft in a known manner until this axially bears on the shaft head (on the underside) in a pivotable and rotatable manner. Subsequently the inlay (above the shaft head) is introduced into the receiving sleeve and its U-shaped slot aligned corresponding to the U-shaped slots in the receiving sleeve. After a longitudinal member/bar has been introduced into the U-shaped slot of the inlay (and inevitably here also into the slots of the receiving sleeve), a first screw (screw sleeve), preferably a grub screw which was previously screwed into the internal thread of the receiving sleeve, is further tightened in order to press the inlay directly against the shaft head and thus to fix the position of the receiving sleeve relative to the shaft. The longitudinal member however remains initially uninfluenced by the first screw, i.e. not fixed. In this assembly state it is possible to apply an adjusting force via the receiving sleeve to the vertebra.
The first screw is likewise sleeve-shaped and has an internal thread into which a second screw (grub screw) is screwed. The longitudinal member is now fixed in the inlay by means of this second screw, in which the second screw is rotated relative to the first screw and presses directly against the bar in order to clamp said bar in the inlay. In this way the polyaxiality of the pedicle screw is locked and therefore the longitudinal member is also fastened.
In the case of many pedicle screws according to the above mentioned construction, the fastening means/locking elements (screws) are essentially self-lockingly executed in order not to risk an undesired loosening of the longitudinal member from the pedicle screws after implanting. In addition the fixing forces between pedicle screw and longitudinal member are considerable because the entire system must withstand large loads without the set positional relationship between shaft, sleeve and longitudinal member being allowed to change. It follows that the clamping forces between the shaft head and the receiving sleeve are likewise very high such that the frictional fit which forms in-between does not frequently come loose even when the fastening means is released. These requirements however also cause problems during the implanting procedure.                As long as the receiving sleeve is not fixedly (by frictional fit) connected with the shaft head, an adjusting force cannot be transmitted via this receiving sleeve to the vertebra. In other words, in the case of a polyaxial pedicle screw according to the one-screw principle for example, a longitudinal member would first have to be inserted and then the locking element, preferably the grub screw, would have to be tightened in order to be able to finally exert an adjusting force on the vertebra. This however would not be technically sensible.        If an operator first of all has tightened the locking element (for example the grub screw) with force and there might arise afterwards the need of a post-adjustment, the positional relationship fixed in this way is not modifiable again, or only with great difficulty. Expressed in other words, for this purpose the operator would have to release again, against their self-locking effect, the grub screw(s) which have been tightened with great force, without releasing or even breaking the externally threaded portion which is already anchored in the vertebra. Even if the grub screw can be released without problems, a possible self-locking frictional fit is configured between the shaft head and the receiving sleeve. This could be released only through significant force effort (impacts on the receiving sleeve, etc.), even in the case of a released grub screw. Furthermore the locking element's self-locking effect is possibly impaired due to its subsequent loosening, such that the operability of the pedicle screw is no longer ensured.        
To this extent, it would be advantageous in principle if the polyaxiality is only provisionally fixable in particular in the case of the one-screw principle, in order to be able to at least temporarily and independently lock the polyaxiality and the bar/longitudinal member. In other words, it would be useful in the case of a polyaxial pedicle screw, also one according to the one-screw principle, to fix by means of a special helping means to be used temporarily only the polyaxiality without the bar being fixed. This would have the advantage that now also the polyaxial pedicle screw would be suited to transmit, before fastening the longitudinal member, correspondingly high forces to the vertebra for its manipulation during the implanting procedure without the inner structure of the pedicle screw becoming significantly more complicated.
It should be noted at this point that the term “provisional”, in particular in view of the invention described below, should not necessarily mean only a light fitting/seizing of the shaft head, rather it is understood in particular to mean such a locking measure which is temporarily used and also whose achievable locking properties such as the clamping force on the shaft head etc. thoroughly correspond to the permanent locking means and can at least approach or even exceed this. Expressed in other words the “provisional” locking means should be dimensioned such that it causes a locking of the receiving sleeve at the shaft head corresponding to the permanent locking element. In this case of an optional subsequent loosening of the frictional fit in accordance with the prior art, it would be possible only though considerable force exertion, however there would be at least the possibility of vertebral readjustment before fastening the longitudinal member. Alternatively, it would also be desirable to dimension the “provisional” locking means such that sufficient adjusting forces are transmittable on the vertebra by means of the polyaxial pedicle screw, wherein the frictional fit achieved here between shaft head and receiving sleeve is releasable again.
However the polyaxial pedicle screws according to the described prior art prove themselves to be unfavourable. Therefore in particular in the case of the one-screw principle the polyaxiality can be locked only when the longitudinal member sits (not yet fixed) in its end position. An inserting of the longitudinal member after the locking of the polyaxiality is actually not possible or is only very difficult and would have as a prerequisite a clearly more complicated construction of the pedicle screw.